The primary purpose of lubrication is separation of solid surfaces moving relative to one another, to minimise friction and wear. The materials most frequently used for this purpose are oils and greases. The choice of lubricant is mostly determined by the particular application.
Lubricating greases are employed where high contact stresses exist, where lubricant leakage from the bearings is undesirable or where the motion of the contacting surfaces is discontinuous so that it is difficult to maintain a separating film in the bearing. Because of design simplicity, decreased sealing requirements and less need for maintenance, greases are almost universally given first consideration for lubricating ball and roller bearings in electric motors, household appliances, automotive wheel bearings, machine tools or aircraft accessories. Greases are also used for the lubrication of small gear drives and for many slow-speed sliding applications.
Lubricating greases consist primarily of a fluid lubricant, such as an oil, and a thickener, typically together with one or more performance additives. Essentially, the same type of oil is employed in compounding a grease as would normally be selected for oil lubrication. Fatty acid soaps of lithium, calcium, sodium, aluminium and barium are commonly used as thickeners. Fatty acid complex soaps are also well known as grease thickeners.
While much is known about the manufacturing processes of lubricating grease compositions, there is a continuing need for improvement so that productivity is maximized and cost is minimized. In particular, reduction of batch times leads to an increase in productivity and reduced cost. There is also a continual need for improving the energy efficiency of grease manufacturing processes.
In the preparation of a grease, for example, a lithium grease, it is required to add a certain amount of water with the raw materials to promote the efficiency of the saponification reaction. The added water reduced the product viscosity in the reactor, helps mixing, increases the reaction at the water/oil interface between raw materials and therefore promotes the chemical reaction. To obtain the correct grease properties, the water must then be removed typically by evaporation in a so-called “venting step”. This venting step is time consuming and energy demanding, particularly in the case of lithium complex greases that contain more solid ingredients and hence need more water. In addition, the higher consistency of lithium complex greases makes the removal of water even more difficult. It would therefore be advantageous to develop a manufacturing process for metal complex greases wherein the amount of water is reduced and the venting time is shortened.
It has now surprisingly been found that by reducing the particle size of the solid components used to make a metal complex grease, e.g. metal base and complexing agent, significantly less water is needed to achieve the saponification reaction, and therefore the venting time is shortened and the grease plant productivity is increased.